The present invention relates to communications systems imploring code division multiple access (CDMA) techniques. More particularly, the present invention relates to a transmission diversity scheme which can be applied to a CDMA communication system.
Spacial diversity has been proposed for support of very high data rate users within third generation wide band code division multiple access systems. Using multiple antennas, the systems achieve better gains and link quality, which results in increased system capacity. Classically, diversity has been exploited through the use of either beam steering or through diversity combining.
More recently, it has been realized that coordinated use of diversity can be achieved through the use of space-time codes. Such systems can theoretically increase capacity by up to a factor equaling the number of transmit and receive antennas in the array. Space-time block codes operate on a block of input symbols producing a matrix output over antennas and time.
In the past, space-time transmit diversity systems have transmitted consecutive symbols simultaneously with their complex conjugates. This type of system, though may result in symbol overlap at the receiving end, with the amount of overlap being dependent on the length of the impulse response of the propagation channel. In time division duplex (TDD) mode, this symbol overlap will have to be accounted for in the joint detection receiver. The joint detector will have to estimate the transmitted symbols and their conjugates, resulting in an increase in complexity of the joint detection.
In order to alleviate this increase in joint detection, systems have been created which transmit two similar but different data fields. The first data field, having a first portion, D11, and a second portion, D12, is transmitted by the first antenna. A second data field is produced by modifying the first data field. The negation of the conjugate of D12, –D12*, is the first portion of the second data field and the conjugate of D11, D11*, is the second portion. The second data field is simultaneously transmitted by the second antenna. This type of system results in the joint detection implemented at the receiver needing only to estimate the same amount of symbols as in the case of a single transmit antenna. A block diagram of this system is illustrated in FIG. 1.
Neglecting the cross interference between the blocks the received signal model can be approximated as
                                          [                                                                                r                    →                                                                                                                                          r                      →                                        2                    *                                                                        ]                    =                                                                      [                                                                                                              A                          11                                                                                                                      -                                                      B                            11                                                                                                                                                                                        B                          22                          *                                                                                                                      A                          22                          *                                                                                                      ]                                                  ︸                  E                                            ·                              [                                                                                                                              d                          1                                                →                                                                                                                                                                          d                          →                                                2                        *                                                                                            ]                                      +                          [                                                                                                                  n                        1                                            →                                                                                                                                                          n                        →                                            2                      *                                                                                  ]                                      ⁢                                  ⁢        where        ⁢                                  ⁢                                            d              ->                        1                    =                                    [                                                S                  1                                ,                                  S                  2                                ,                ⋯                ⁢                                                                  ,                                  S                                      N                    /                    2                                                              ]                        T                          ⁢                                  ⁢                                            d              ->                        2                    =                                    [                                                S                                                            N                      /                      2                                        +                    1                                                  ,                                  S                                                            N                      /                      2                                        +                    2                                                  ,                ⋯                ⁢                                                                  ,                                  S                  N                                            ]                        T                                              Equation        ⁢                                  ⁢        1            are the vector forms of the transmit symbol sequences. Aij and Bij are the sub-matrices of the banded propagation matrices A and B according to the channels from antenna 1 and 2 to a specific user respectively. They are rewritten by the following (2×2) block matrix representations:
      A    =          [                                                                  A                11                            ⁢              O                                                                                          A                21                            ⁢                              A                22                                                        ]        ,          ⁢      B    =                  [                                                                              B                  11                                ⁢                O                                                                                                          B                  21                                ⁢                                  B                  22                                                                    ]            .      Each column of the matrices A and B is the shifted versions of the convolution of the spreading code and the channel impulse response from the first and diversity antennas respectively.
The model of Equation 1 can be solved using a MMSE BLE by
                                          [                                                                                                      d                      →                                                              mmse                      ⁢                                                                                          ⁢                      1                                                                                                                                                              d                      →                                                              mmse                      ⁢                                                                                          ⁢                      2                                        *                                                                        ]                    =                                    D                              -                1                                      ·                                          E                H                            ⁡                              [                                                                                                                              r                          1                                                →                                                                                                                                                                          r                          →                                                2                        *                                                                                            ]                                                    ,                                  ⁢                              with            ⁢                                                  ⁢            D                    =                                                    E                H                            ⁢              E                        +                                          σ                n                2                            ⁢              I                                                          Equation        ⁢                                  ⁢        2            where σn2 is the variance of the additive white Gaussian noise. It can be simplified using the sub-block matrix manipulations and banded Toeplitz approximations.
The problem with the above-transmit diversity system is that the first and second portion, D11, D12 of the first data field requires the same number of symbols in each of the portions. Some TDD data fields include an odd number of symbols. Therefore, when the data field is split into two portions, the portions have a different number of symbols. A method to deal with this inequality must be implemented. One approach duplicates the first symbol to alleviate this problem. Other approaches are known in the art. Utilizing one of these methods results in additional computations for joint detection at the receiver. In particular, the first symbol is not STTD encoded, and hence the STTD encoder output becomes:S1,−(S(N+1)/2+1,S(N+1)/2+2, . . . SN)*, (S2,S3, . . . ,S(N+1)/2)*
Furthermore, the initial approximation by eliminating the center elements of Equation 1 introduces a small error in the joint detection process.
Accordingly, there exists a need for other transmit diversity systems.